A. Identification of receptors for GI peptides. We have recently discovered a specific high affinity receptor for the bombesin (Bn) related peptide neuromedin B (NMB) and developed specific ligands for it. In collaboration with T.W. Moody and T.H. Moran, these receptors have been identified to be widely distributed in the central nervous system. This receptor was cloned by J. Battey this year and from collaborative experiments pharmacologically characterized as distinct from the receptor for the mammalian bombesin-related peptide gastrin-releasing peptide (GRP). Because of the recent evidence that GI peptides may act as tumor growth factors we have investigated the occurrence of receptors for these on 10 human colon cell lines. We have demonstrated that of 12 different hormone and neuro-transmitters examined, each human colon cancer cell line possesses receptors for at least one of these agents. The receptors were pharmacologically characterized and occupation by agents was shown to alter intracellular mediators. These studies are now being extended to determine whether occupation of these receptors affects tumor growth. In collaboration with R.H. Bell azaserine-induced neoplasms were shown to overexpress receptors for cholecystokinin (CCK). This finding may be important in the pathogenesis of these tumors because CCK-related peptides increase the rate of their development through an unknown process. B. Characterization of receptors for gastrointestinal hormones by the development of specific antagonists. In collaborative studies with D.H. Coy we have extended our previous studies attempting to understand the active configuration of Bn related peptides to alter cellular function and to develop more potent antagonists. Our previous studies led us to propose that the COOH terminus of bombesin might adopt a beta turn similar to proposed for LHRH and somatostatin. This conclusion has been supported during the year by molecular modeling studies done in collaboration by D.H. Coy as well as the synthesis of cyclic Bn analogues that are conformationally restricted and yet function as agonists and others as antagonists. Our previous study showed that various des Met14 analogue of bombesin were extremely potent antagonists in vitro and in vivo. To improvement potentcy [D-Ala11] Bn analogues were synthesized to decrease degradation and a pentafluoro phenylalanine group added to increase lipophilicity and the analogue [2,3,4,5,6 pentafluoro Phe6 D-Ala11] Bn (6- 13) methyl ester was found to have a duration of action in vivo of >10 fold that seen with other Bn receptor antagonists.